In many types of operations conducted in an oil well or well bore, instrumentation is lowered into the well bore on either an electrical wireline, a slickline or tubing to obtain measurements of one or more down hole parameters such as pressure or temperature as a function of time. Typically, these measurements are transmitted (where a wireline cable is involved) to the surface of the earth for processing, recording and subsequent analysis. Similarly, electrical power requirements for the instrumentation are transmitted from the surface to the down hole instrumentation. In many instances, however, it is desired to have a down hole, self-contained power system for operation of the down hole instrumentation and recording data relative to the measurement parameters. In still other instances, the down hole instrumentation may be left in the well bore for a prolonged period of time to obtain a series of down hole measurements over the prolonged period of time for evaluation purposes. To obtain the down hole power required for the instrumentation it is common to use D.C. batteries (dry cells) which can be specially manufactured or purchased from commercially available battery sources.
In production logging types of equipment for oil field use it is common to take down hole pressure and temperature measurements over a period of time and typically such instrumentation is in a small diameter range such that size C batteries and sometimes smaller batteries are used for the down hole D.C. power supply. The tool configuration typically has an outer diameter sized to pass through a production tubing. As a consequence, the batteries are typically series connected in the tool housing similar to a flashlight battery connection.
Where high temperatures are involved (say above 212.degree. F.), the commercially available batteries are silver oxide and are relatively expensive, a size C battery presently costing about $60.00 a unit. A typical D.C. battery is rated in terms of ampere-hour life for a given voltage. In other words, the battery will provide a certain amount of current for a certain amount of time at a certain voltage before it expires. However, one of the problems in a D.C. battery is the fact that as the battery life is being used, the voltage output capacity of the battery tends to decrease.
In any event, in a typical oil field production test of temperature and pressure, the cost of the operation relative to the cost of batteries is high. As a consequence, it is common to use a fresh set of batteries for each independent operation so that the reliability of the tool during the time required for the down hole measurements is assured. This is a safety factor since used batteries in subsequent use may become defective during the operation. Also, recycling of batteries between high temperature and lower temperatures for different operations tends to cause battery failure when subsequently used. As a consequence, a new set of batteries is used for each operation and this can be a significant and expensive operating cost for use of the equipment and increases the overall cost of the operation.
The present invention concerns itself with a system for minimizing the number of D.C. batteries required in a self-contained down hole tool to provide adequate voltage and current for the measurement of time. In the use of the system, the number of batteries used is the minimum required for a given operation and the batteries will be completely utilized in a manner which compensates for the loss of voltage over the life of the battery and yet maintains a regulated voltage output for operation of equipment for a defined period of time.
Prior art patents which are related to the type of equipment contemplated by the present invention are:
______________________________________ U.S. Pat. No. Issued Inventor ______________________________________ 4,033,186 7/05/77 Bresie 4,161,782 7/17/79 McCracken ______________________________________